U.S. patent number 8,017,825 [Application Number 12/316,757] was granted by the patent office on 2011-09-13 for modified ets-10 zeolites for olefin separation.
This patent grant is currently assigned to The Governors of the University of Alberta. Invention is credited to Alejandro Anson, Steven M. Kuznicki, Christopher C. H. Lin, Tetyana Segin.
United States Patent |
8,017,825 |
Kuznicki , et al. |
September 13, 2011 |
Modified ETS-10 zeolites for olefin separation
Abstract
An as prepared Na-ETS-10 zeolite was modified by ion exchange
with a mono-, di-, or tri-valent cation and mixtures thereof.
Several of the modified ETS-10 zeolites showed improved pressure
swing capacity during the selective adsorption of ethylene from an
ethylene/ethane mixture, relative to Na-ETS-10, although the
selectivity of adsorption decreased. Modification with Ba.sup.2+
and Ba.sup.2+/H.sup.+ provided modified ETS-10 zeolite adsorbents
having a good balance of selectivity and pressure swing capacity
for the separation of ethylene/ethane mixtures, making them useful
adsorbents for PSA processes.
Inventors: |
Kuznicki; Steven M. (Edmonton,
CA), Anson; Alejandro (Zaragoza, ES),
Segin; Tetyana (Edmonton, CA), Lin; Christopher C.
H. (Edmonton, CA) |
Assignee: |
The Governors of the University of
Alberta (Alberta, CA)
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Family
ID: |
40877001 |
Appl.
No.: |
12/316,757 |
Filed: |
December 16, 2008 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20090187053 A1 |
Jul 23, 2009 |
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Foreign Application Priority Data
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Jan 21, 2008 [CA] |
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2618267 |
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Current U.S.
Class: |
585/829;
208/310Z; 585/820 |
Current CPC
Class: |
B01D
53/02 (20130101); C07C 7/13 (20130101); C01B
39/026 (20130101); C07C 7/13 (20130101); C07C
11/04 (20130101); B01D 2253/108 (20130101); B01D
2257/7022 (20130101); B01D 53/047 (20130101); Y02P
20/52 (20151101) |
Current International
Class: |
C07C
7/13 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Kotelnikov, et al, Application of FBD process for C3-C4 olefins
production from light paraffins, Stud. Surf. Sci. Catal., 2004, pp.
67-72, v147. cited by other .
Paul F. Bryan, Removal of Propylene fron Fuel-Grade Propane, Sep.
Purif. Rev., 2004, pp. 157-182, vol. 33, No. 2. cited by other
.
Kirk-Othmer Encyclopedia of Chemical Technology, Adsorption, Gas
Separation, 2001, vol. 1, pp. 617-647, John Whiley & Sons,
Inc., Online Edition. cited by other .
Choudary, et al, Adsorption of Light Hydrocarbon Gases on
Alkene-Selective Adsorbent, Ind. Eng. Chem. Res., 2002, 41,
2728-2734. cited by other .
Miltemburg, et al, Adsorption of Light Olefin/Paraffin Mixtures,
Chem. Eng. Res. and Des. May 2006, 84(A5), pp. 350-354. cited by
other .
Al-Baghli, et al, Binary and Ternary Adsorption of Methane, Ethane,
and Ethylene on Titanosilicate ETS-10 Zeolite, J. Chem. Eng. Data,
2006, 51, 248-254. cited by other .
Al-Baghli, et al, Adsorption of Methane, Ethane, and Ethylene on
Titanosilicate EST-10 Zeolite, J. Chem. Eng. Data. 2005, 50,
843-848. cited by other .
Rouquerol, et al, Adsorption by Powders and Porous Solids, 1999,
pp. 1-26 and 165-189, Academic Press, San Diego, California. cited
by other .
Myers, et al, Thermodynamics of Mixed-Gas Adsorption, 1965, pp.
121-127, A.I.Ch.E. Journal, vol. 11, No. 1. cited by other .
Valenzuela, et al, "Caluclations fo Mixed-Gas Adsorption from
Single-Gas Isotherms", Adsorption Equilibrium Data Handbook, 1989,
pp. 208-217, Prentice Hall, Englewood, N.J. cited by other .
Greg, et al, "The Use of Gas Adsorption for the Determination of
Surface Area and Pore Size Distribution", Adsoprtion, Surface Area
and Porosity, 1982, pp. 283-286, Academic Press, London-New York.
cited by other.
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Primary Examiner: Nguyen; Tam M
Attorney, Agent or Firm: Johnson; Kenneth H
Claims
What is claimed is:
1. A method of increasing the proportion of olefin in a gaseous
mixture comprising said olefin and a paraffin having the same
number of carbon atoms as said olefin, wherein said process
comprises: passing said mixture over a modified ETS-10 zeolite at a
temperature and pressure wherein said modified ETS-10 material
selectively adsorbs said olefin from said mixture, and lowering the
pressure and optionally increasing the temperature to release the
selectively adsorbed olefin from said modified ETS-10 zeolite.
2. The method of claim 1, wherein said olefin is ethylene and said
paraffin is ethane.
3. The method of claim 2, wherein said modified ETS-10 zeolite
comprises an as prepared Na-ETS-10 zeolite which has been modified
by cation exchange with one or more than one mono-, di- or
tri-valent cation or mixtures thereof.
4. The method of claim 3, wherein said as prepared Na-ETS-10
zeolite comprises pores having a size of about 8 Angstroms.
5. The method of claim 4, wherein said as prepared Na-ETS-10
zeolite has an oxide mole ratio which is represented by: x
M.sub.2O:TiO.sub.2:y SiO.sub.2:z H.sub.2O, wherein M is Na.sup.+ or
both Na.sup.+ and K.sup.+, x is from 1 to 10, y is from 2.5 to 25,
and z is from 0 to 150.
6. The method of claim 5, wherein said modified ETS-10 zeolite
comprises an as prepared Na-ETS-10 zeolite which has been modified
with one or more than one mono-, di- or tri-valent metal cation, a
proton or mixtures thereof.
7. The method of claim 6, wherein said metal cation is selected
from the group consisting of Li.sup.+, Cu.sup.+, Ba.sup.2+,
Sr.sup.3+, La.sup.3+ and mixtures thereof.
8. The method of claim 6, wherein said modified ETS-10 zeolite
comprises an as prepared Na-ETS-10 that has been modified with
Ba.sup.2+, or both Ba.sup.2+ and H.sup.+ or both La.sup.3+ and
H.sup.+.
9. The method of claim 8, wherein the adsorption and release of
ethylene is carried out at ambient temperature.
10. A method for the selective sequestration of ethylene from a
gaseous mixture comprising ethylene and ethane, wherein said method
comprises: passing said mixture over an adsorbent which selectively
adsorbs ethylene from said mixture, said adsorbent comprising an as
prepared Na-ETS-10 zeolite which has been modified by cation
exchange with one or more than one mono-, di- or tri-valent cation
or mixtures thereof.
11. A pressure swing adsorption process for increasing the
proportion of an olefin in a gaseous mixture comprising said olefin
and a paraffin having the same number of carbon atoms as said
olefin, wherein said process comprises: passing said mixture
through a bed comprising a modified ETS-10 zeolite at a pressure at
which the bed selectively adsorbs said olefin to give a waste or
recycle stream enriched in paraffin; reducing the pressure in said
bed to a pressure at which the bed releases said adsorbed olefin to
give a product stream enriched in olefin; wherein said modified
ETS-10 zeolite comprises an as prepared Na-ETS-10 zeolite which has
been modified by cation exchange with one or more than one mono-,
di- or tri-valent cation or mixtures thereof.
12. The process of claim 11, wherein said olefin is ethylene and
said paraffin is ethane.
13. The process of claim 12, wherein the process is carried out at
ambient temperature.
14. A process for the selective removal of ethylene from a gaseous
mixture comprising ethylene and ethane, said gaseous mixture being
a product feedstream from an ethane hydrocracking unit, wherein
said process comprises: passing said mixture over an adsorbent
which selectively adsorbs ethylene from said mixture, said
adsorbent comprising an as prepared Na-ETS-10 zeolite which has
been modified by cation exchange with one or more than one mono-,
di- or tri-valent cation or mixtures thereof.
15. A process for the separation of an olefin from a mixture
comprising said olefin and a paraffin having the same number of
carbon atoms as said olefin, wherein said mixture is subjected to
both cryogenic distillation and pressure swing adsorption (PSA),
said PSA comprising: passing said mixture through one or more PSA
beds containing a modified ETS-10 zeolite which selectively adsorbs
said olefin; and regenerating said one or more PSA beds to produce
a product stream enriched in said olefin.
16. The process of claim 15, wherein said one or more PSA units
increase the ethylene concentration in a C2 product stream
immediately upstream or immediately downstream of a C2 splitter
column; said C2 splitter column receiving the C2 product stream in
a hydrocarbons cracking plant.
17. A method to adjust the adsorption selectivity and the pressure
swing capacity of an as prepared Na-ETS-10 zeolite for use in a
pressure swing adsorption (PSA) separation of ethylene and ethane,
said method comprising: modifying said as prepared Na-ETS-10
zeolite by cation exchange with one or more than one mono-, di- or
tri-valent cation or mixtures thereof.
18. The method of claim 1, wherein said modified ETS-10 zeolite
comprises a structural variant of an as prepared Na-ETS-10 zeolite,
wherein said structural variant comprises: a crystalline molecular
sieve having a pore size of at least 8 Angstrom units and having a
composition consisting of in terms of mole ratios of oxide as
follows: a(1.0.+-.0.25)M.sub.2/nO:AO.sub..alpha.:d
BO.sub..beta.:0-100 H.sub.2O wherein A is octahedrally coordinated
titanium alone or a mixture of octahedrally coordinated titanium
and another octahedrally coordinated metal selected from the group
consisting of arsenic, cobalt, chromium, copper, iron, germanium,
hafnium, magnesium, manganese, molybdenum, niobium, nickel,
antimony, tin, uranium, vanadium, yttrium, zinc, zirconium,
lanthanum, an actinide and a lanthanide and mixtures thereof; B is
silicon alone or a mixture of silicon and another metal selected
from the group consisting of aluminum, arsenic, bismuth, boron,
beryllium, cobalt, chromium, copper, iron, gallium, germanium,
indium, lead, magnesium, manganese, molybdenum, niobium, nickel,
antimony, tin, titanium, vanadium, tungsten, zinc, and mixtures
thereof; M is at least one cation of valence n; .alpha. is 1/2 the
valence of A; .beta. is 1/2 the valence of B; d is 2-100; a is
equal to 1/2 the charge provided by the total of A and B with the
proviso that when A is solely titanium, B cannot be solely silicon
and that when B is solely Si, A cannot be solely Ti.
Description
FIELD OF THE INVENTION
The present invention relates to the adsorptive separation of
olefins from paraffins. Specifically, a cation modified, large pore
titanosilicate having a good pressure swing capacity, is used to
selectively adsorb ethylene form a mixture of ethylene and ethane
at ambient temperatures.
BACKGROUND OF THE INVENTION
The commercial production of olefins such as ethylene and propylene
relies mainly on the pyrolysis of light hydrocarbon feeds at high
temperatures. Thermal cracking of ethane, propane or higher
hydrocarbons invariably leaves un-cracked paraffins and other
undesirable compounds in the product stream. The undesirable
paraffins (e.g. ethane, propane etc.) must be separated from
ethylene, propylene and other products which, due to the similar
boiling points of paraffins and olefins having the same carbon
number, requires the use of energy intensive cryogenic distillation
columns. Such "superfractionations" represent a significant portion
of the cost associated with running a cracking unit. Specifically,
it would be beneficial if expensive C2 or C3 splitter columns could
be augmented or replaced.
In the interests of reducing cost and operating complexity, several
methods have been explored to replace the expensive separation
processes used in traditional hydrocracking plants. These include
selectively adsorptive membranes (see for example U.S. Pat. Nos.
6,395,067; 6,340,433; Kotelnikov et al. in Stud. Surf. Sci. Catal.
2004, v 147, p 67 and Bryan et al. in Sep. Purif. Rev. 2004, v 33,
p 157), liquid extraction systems, and pressure swing adsorption
methods (see for example U.S. Pat. Nos 3,430,418; 4,589,888; and
6,497,750).
Pressure swing adsorption (PSA) processes generally include i) a
high pressure adsorption step, during which a component in a
gaseous mixture is selectively adsorbed onto an adsorbent substrate
ii) a purging step, during which non-adsorbed components are
collected as waste, recycle or product effluent; and iii) a low
pressure de-sorption step or regeneration step, during which the
selectively adsorbed component is released form the adsorbent
substrate (see for example, U.S. Pat. No. 6,197,092 that is
incorporated herein by reference). In a PSA processes, the
adsorbent material is typically packed in one or more beds, and
various pressurization/depressurization protocols including the
application of vacuum can be used (see Adsorption, Gas Separation
in the Kirk-Othmer Encyclopedia of Chemical Technology, Copyright
John Wiley & Sons, Inc. vol 1, pg 617 and references cited
therein).
Several types of adsorbents have been developed for the separation
of various gas mixtures by PSA processes, and the useful
application of each depends mainly of the nature of the gases to be
separated. PSA, and similar separation processes such as thermal
swing adsorption (TSA), may utilize a kinetically effected
separation, which excludes one potential adsorbent due to pore
diameter restrictions in the adsorbent, and/or thermodynamically
effected separation, in which one potential adsorbate binds more
strongly to the adsorbent than another potential adsorbate under
equilibrium conditions. Thermodynamic separations may be
facilitated by electrostatic or bonding interactions between an
adsorbent material and an adsorbate molecule.
Adsorbents for the separation of olefins from paraffins often
include high surface area, porous materials which have been treated
with metal species capable of .pi.-complexation with olefins, such
as copper and silver salts. For example, U.S. Pat. No. 4,917,711
describes the use of supports such as zeolite 4A, zeolite X,
zeolite Y, alumina and silica, each treated with a copper salt, to
selectively remove carbon monoxide and/or olefins from a gaseous
mixture containing saturated hydrocarbons (i.e. paraffins) such as
ethane and propane.
U.S. Pat. Nos 6,867,166 and 6,423,881 describe the use of copper
salts and silver compounds supported alternatively on silica,
alumina, MCM-41 zeolite, 4A zeolite, carbon molecular sieves,
polymers such as Amerberlyst-35 resin, and alumina to selectively
adsorb olefins from gaseous mixtures containing olefins and
paraffins. Both kinetic and thermodynamic separation behavior was
observed and modeled.
Clay based adsorbents which have been treated with silver salts are
taught by Choudary et al. in the Ind. Eng. Chem. Res. 2002, v 41, p
2728. The article describes Ag.sup.+ impregnated clay adsorbents
that are selective for olefin uptake from a gaseous olefin/paraffin
mixture. Up to 20% of the olefin is adsorbed in an irreversible
manner. The adsorbent was evaluated for its performance in a four
bed vacuum swing adsorption process. Ethylene was separated from
ethane with over 85% recovery and in over 99% purity.
An article in Chemical Engineering Research and Design, 2006,
84(A5) p 350, by Van Miltenburg et al. reported the use of Cu.sup.+
to modify Faujasite zeolites. The modified zeolites were useful
adsorbents for the separation of ethylene from ethylene/ethane
mixtures. The use of similarly modified Faujasite zeolites in a
highly selective PSA process that separates carbon monoxide and/or
olefins from a mixture that also contained paraffins was reported
in U.S. Pat. No. 4,717,398 assigned to BP.
In U.S. Pat. Nos 5,744,687; 6,200,366 and 5,365,011 assigned to
BOC, copper modified 4A zeolites were used to separate ethylene and
propylene form ethane and propane respectively. Elevated
temperatures were required for successful application to PSA
processes (i.e. from 50.degree. C. to 200.degree. C.). Zeolites
such as zeolite 5A and zeolite 13X were also used in the formation
of copper modified adsorbents.
U.S. Pat. No. 6,293,999 assigned to UOP, describes the use of
aluminophosphates to separate propylene from propane in a PSA
process. The aluminophosphate is a small pore molecular sieve
designated "AIPO-14". The system operates at temperatures of from
25.degree. C. to 125.degree. C. to effect a kinetic separation of
propylene from propane. U.S. Pat. No. 6,296,688 also to UOP,
discloses a vacuum swing adsorption process for separating
propylene form propylene/propane mixtures using analogous zeolite
adsorbents.
Despite the above progress, new materials having high selectively
and good pressure swing capacity are still needed for
olefin/paraffin separation processes. Particularly desirable are
adsorbents that can be tuned to suit commercial process conditions
or adsorbents that are effective in ambient temperature PSA
separation of olefin/paraffin mixtures.
In U.S. Pat. Nos 4,938,939 and 5,011,591, assigned to Engelhard
Corp., a new family of crystalline titanium silicate zeolite
materials was disclosed.
U.S. Pat. No. 4,938,939, describes a small pore zeolite, designated
"ETS-4" with pore diameters of about 3-5 .ANG.. Modification of the
ETS-4 materials by cation exchange with for example, Ba.sup.2+ and
Sr.sup.3+ gave adsorbents which were useful in the separation of
nitrogen from methane using PSA processes (see U.S. Pat. Nos
6,068,682 and 5,989,316).
As described in U.S. Pat. No. 6,517,611, heat treatment of ETS-4
gave a controlled pore volume zeolite material, dubbed "CTS-1"
which is a highly selective absorbent for olefin/paraffin
separations. The CTS-1 zeolite, which has pore diameters of from
about 3-4 .ANG., selectively adsorbed ethylene from a mixture of
ethylene and ethane through a size exclusion process. The pore
diameter of CTS-1, allowed diffusion of ethylene, while blocking
diffusion of ethane which was too large to enter the pores of the
CTS-1 zeolite, thereby providing a kinetic separation. The CTS-1
adsorbent was successfully applied to a PSA process in which
ethylene or propylene could be separated from ethane or propane
respectively.
U.S. Pat. No. 5,011,591 discloses the synthesis of a large pore
diameter titanosilicate designated "ETS-10". In contrast to ETS-4
and CTS-1, the large pore titanosilicate material, ETS-10, which
has pore diameters of about 8 .ANG., cannot kinetically distinguish
light olefins from paraffins of the same carbon number.
Nevertheless, high degrees of selectivity have been reported for
the separation of ethylene from ethane using as prepared ETS-10
zeolites; see: Al-Baghli and Loughlin in J. Chem. Eng. Data 2006, v
51, p 248. The authors demonstrate that Na-ETS-10 is capable of
selectively adsorbing ethylene from a mixture of ethylene and
ethane under thermodynamic conditions, even at ambient temperature.
Although, the reported selectivity for ethylene adsorption using
Na-ETS-10 was high at ambient temperature, the adsorption isotherms
for ethylene and ethane had highly rectangular shapes consistent
with a low pressure swing capacity. Consequently, Na-ETS-10 is not
readily applicable to pressure swing absorption processes (PSA), at
least at lower or ambient temperatures.
We have now found that the separation selectivity and pressure
swing capacity of Na-ETS-10 can be dramatically affected by cation
exchange. The resulting modified ETS-10 zeolites provide more
useful pressure swing capacities for olefin/paraffin separation. In
addition, the modified ETS-10 zeolites can be precisely tuned by
cationic exchange to cover a range of adsorbent behavior from
silica type adsorbents (i.e. weak adsorbents) to more traditional
zeolites (i.e. strong adsorbents). Hence, the ETS-10 zeolites can
be modified to suit a wide range of PSA process conditions for the
separation of olefins from paraffins and in some cases are suitable
for ambient temperature PSA.
SUMMARY OF THE INVENTION
Provided is a method for the selective sequestration of ethylene
from a gaseous mixture comprising ethylene and ethane, wherein said
method comprises: passing said mixture over an adsorbent which
selectively adsorbs ethylene from said mixture, said adsorbent
comprising a modified ETS-10 zeolite.
The present invention provides a method of increasing the
proportion of olefin in a gaseous mixture comprising said olefin
and a paraffin having the same number of carbon atoms as said
olefin, wherein said process comprises: (a) passing said mixture
over a modified ETS-10 zeolite at a temperature and pressure
wherein said modified ETS-10 material selectively adsorbs said
olefin from said mixture, and (b) lowering the pressure and
optionally increasing the temperature to release the selectively
adsorbed olefin from said modified ETS-10 zeolite.
The invention improves the applicability of large pore
titanosilicate zeolites to PSA processes which separate olefins and
paraffins of the same carbon number by increasing the pressure
swing capacity of the zeolites though cation exchange
modification.
The present invention also teaches the use of structural variants
of unmodified ETS-10 as adsorbents for the selective uptake of
olefins from a gaseous mixture of olefins and paraffins having the
same number of carbon atoms.
The current invention provides a cationically modified ETS-10
zeolite having a superior pressure swing capacity relative to
unmodified ETS-10 for the separation of olefins and paraffins
having the same number of carbon atoms.
The current invention provides a cationically modified ETS-10
zeolite having a superior pressure swing capacity in the pressure
range of about 1 kPa to about 200 kPa, relative to unmodified
ETS-10 for the separation of olefins and paraffins having the same
number of carbon atoms.
In an aspect of the invention, a cationically modified ETS-10
zeolite with attenuated selectivity but with improved pressure
swing capacity for application to the separation of ethylene from
ethane in pressure cycling processes, is provided.
In an aspect of the invention, as prepared Na-ETS-10 is modified by
a mono-, di, or tri-valent cation to provide a modified ETS-10
which selectively absorbs olefins from a mixture of olefins and
paraffins having the same number of carbon atoms.
In another aspect of the invention, cationic modification of as
prepared Na-ETS-10 provides an adsorbent for the PSA separation of
olefins and paraffins having the same number of carbon atoms, at
ambient temperatures.
The current invention provides a method to adjust the adsorption
selectivity and the pressure swing capacity of an as prepared
Na-ETS-10 zeolite for use in a pressure swing adsorption (PSA)
separation of ethylene and ethane, said method comprising:
modifying said as prepared Na-ETS-10 zeolite by cation exchange
with one or more than one mono-, di- or tri-valent cation or
mixtures thereof.
A pressure swing adsorption process is provided for increasing the
proportion of an olefin in a gaseous mixture comprising said olefin
and a paraffin having the same number of carbon atoms as said
olefin, wherein said process comprises: (a) passing said mixture
through a bed comprising a modified ETS-10 zeolite at a pressure at
which the bed selectively adsorbs said olefin to give a waste or
recycle stream enriched in paraffin; (b) reducing the pressure in
said bed to a pressure at which the bed releases said adsorbed
olefin to give a product stream enriched in olefin; wherein said
modified ETS-10 zeolite comprises an as prepared Na-ETS-10 zeolite
which has been modified by cation exchange with one or more than
one mono-, di- or tri-valent cation or mixtures thereof.
In an aspect of the current invention, the molecular sieve "ETS-10"
is modified to control its selectivity towards ethane and ethylene
binding for the partial separation of the same under desired
process conditions. The modified ETS-10 zeolites can be used in PSA
adsorption processes which when combined with cryogenic
distillation, can reduce the energy requirements and manufacturing
costs in the manufacture of ethylene from ethane by a thermal
hydrocracking processes.
A process is provided for the selective removal of ethylene from a
gaseous mixture comprising ethylene and ethane, said gaseous
mixture being a product feedstream from an ethane hydrocracking
unit, wherein said process comprises: passing said mixture over an
adsorbent which selectively adsorbs ethylene from said mixture,
said adsorbent comprising an as prepared Na-ETS-10 zeolite which
has been modified by cation exchange with one or more than one
mono-, di- or tri-valent cation or mixtures thereof.
A process is provided for the separation of an olefin from a
mixture comprising said olefin and a paraffin having the same
number of carbon atoms as said olefin, wherein said mixture is
subjected to both cryogenic distillation and pressure swing
adsorption (PSA), said PSA comprising: (a) passing said mixture
through one or more PSA units containing a modified ETS-10 zeolite
which selectively adsorbs said olefin, and (b) regenerating said
one or more PSA units to produce a product stream enriched in said
olefin.
In another embodiment of the invention, one or more PSA units
containing a modified ETS-10 zeolite increase the ethylene
concentration in a C2 product stream immediately upstream or
immediately downstream of a C2 splitter column; said C2 splitter
column receiving the C2 product stream in a hydrocarbons cracking
plant.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1a, shows ethylene (open circles) and ethane (shaded circles)
adsorption isotherms at 25.degree. C. for an unmodified Na-ETS-10
zeolite. Dotted lines represent the constrained Toth model
isotherms.
FIGS. 1b-1h, show ethylene (open circles) and ethane (shaded
circles) adsorption isotherms at 25.degree. C. for a series of
cationically modified ETS-10 zeolites prepared according to the
current invention. Dotted lines represent the constrained Toth
model isotherms.
FIGS. 2a and 2b show the ideal adsorption solution theory (IAST)
selectivity at 25.degree. C. for a binary ethylene/ethane mixture
(y=0.5) as a function of the total pressure for Na-ETS-10 and a
series of cationically modified ETS-10 zeolites prepared according
to the current invention.
FIG. 3 shows the IAST plot of y.sub.ethylene vs. x.sub.ethylene at
25.degree. C. and a total pressure of 150 kPa for unmodified (a)
Na-ETS-10 and cationically modified, (b) K, (c) Li, (d) Cu and Ba
(e) Ba/H and (f) La/H, ETS-10 zeolites prepared according to the
current invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The present invention relates to the use of adsorbents comprising
modified titanium silicate molecular sieves for the separation of
olefins from a mixture of olefins and paraffins having the same
number of carbon atoms.
As used herein the term "olefin" refers to .alpha.-olefins or
"alpha" olefins and connotes a terminal olefin or a 1-olefin, in
which the double bond resides only at the terminal position (as
opposed to an internal olefin, in which the double bond is at an
internal site). Some specific olefins include but are not limited
to ethylene, propylene, and 1-butene. The term "paraffin" refers to
any fully saturated hydrocarbons and includes but is not limited to
ethane, propane, n-butane, n-pentane and the like. In the current
invention the olefins and paraffins are preferably gaseous under
the process conditions and have from 2 to 6 carbon atoms, with
ethane and ethylene being the most preferred olefin and paraffin
respectively.
As used herein, the term "modified" encompasses cationic
modification and structural modification (or structural variation)
of an as prepared ETS-10 zeolite.
As used herein the term "cationic modifier" represents a cation,
typically delivered in the form of a salt or acid, which when added
to an unmodified ETS-10 zeolite, provides a modified ETS-10 zeolite
through cation exchange reactions.
As used herein the term "structural modifier" represents a
compound, which when added to an unmodified ETS-10 zeolite,
provides a modified ETS-10 zeolite through substitutions of Ti
and/or Si sites or through extraction of a portion of the titanium
present. Structural modifiers can also be added during the
synthesis of an unmodified ETS-10 to give instead a modified ETS-10
zeolite.
As used herein, the term "pore diameter" refers to the effective
diameter of the largest gas molecules that are significantly
adsorbed by the ETS-10 zeolite materials. This may be similar to,
but different from the crystallographically determined pore
diameter of the ETS-10 zeolite material.
As used herein the terms "separate" or "separation" as well as
"selective removal" connote a partial or full separation of at
least one component in a gaseous mixture. Hence at least one
component may be completely removed or isolated (i.e. purity of 90%
or higher) or merely enriched (i.e. the concentration or proportion
of a component in a gaseous mixture is increased beyond its initial
value) during the process of the current invention.
As used herein the term "pressure swing capacity" has its
conventional meaning and generally refers to the amount (in
millimoles per gram, mmol/g) of gaseous component (i.e. an olefin
or a paraffin) that can be adsorbed on and desorbed from an
adsorbent, between a first higher pressure and a second lower
pressure respectively. In the current invention, the pressure swing
capacity is reported as the amount of a gaseous component that can
be absorbed and/or desorbed over a given pressure range at a given
temperature as indicated by an adsorption isotherm. It will be
obvious to a person skilled in the art, that a "swing capacity" for
a temperature range can be similarly defined.
Unmodified or "as prepared" ETS-10 zeolites which are herein
designated "Na-ETS-10" zeolites, mainly contain Na.sup.+ as
exchangeable counterions but in some cases, depending on
preparation conditions, may also contain exchangeable K.sup.+
counterions. The unmodified titanium silicate molecular sieves
(i.e. Na-ETS-10) of the current invention have octahedral titanium
sites and tetrahedral silicon sites, providing a structure with an
average pore diameter of approximately 8 .ANG. and a titania/silica
mol ratio of from 2.5 to 25. A non-limiting description of
unmodified ETS-10 zeolites is given in Table 1 of J. Chem. Eng.
Data. 2005, 50, p 843 by Al-Baghli et al. that is incorporated
herein by reference.
The "modified" ETS-10, titanium silicates are derived from "as
prepared" or unmodified ETS-10 zeolites through cation exchange
reactions and/or structural exchange reactions. Alternatively,
modified ETS-10 zeolites may be obtained by manipulation of the
preparative recipe and conditions used for making Na-ETS-10. All
such modifications are contemplated by the current invention,
provided that the modified ETS-10 zeolite remains selective for the
adsorption of olefins over paraffins.
In the current invention, the terms "modified" or "modified ETS-10
zeolite" connote an Na-ETS-10 zeolite in which at least some of the
exchangeable Na.sup.+ and/or K.sup.+ ions originally present in the
zeolite are replaced by other cationic species by cationic exchange
reactions. Such modifications are "cationic modification(s)". The
terms "modified" or "modified ETS-10 zeolite" also connote a
titanium silicate zeolite which differs from an as prepared
Na-ETS-10 zeolite by one or more substitutions at the octahedral
titanium sites or the tetrahedral silicon sites (i.e. a structural
variant of Na-ETS-10 in which a partial exchange of Ti and/or Si
has occurred). Such Ti and/or Si substitutions are structural in
nature and for the purposes of the current invention are designated
"structural modification(s)". Hence, in the current invention, the
terms "modified" or "modified ETS-10 zeolites" includes ETS-10
zeolites that have either or both of: i) substitution of
exchangeable cations (e.g. Na.sup.+ and/or K.sup.+ sites); ii)
substitution at the titanium and/or silicon sites.
By way of non-limiting example, an Na-ETS-10 can be cationically
modified by stirring the Na-ETS-10 zeolite with a suitable 3ation
source, to exchange some of the exchangeable cations originally
present in the Na-ETS-10.
Structural variations to the Ti or Si sites of Na-ETS-10 can be
achieved by modifying or changing the source components used to
make the Na-ETS-10. Structural modification can also be achieved
though use of exchange reactions where the Ti and/or Si sites of
"as prepared" Na-ETS-10 are substituted by suitable metal species,
after the Na-ETS-10 material is isolated. Both types of structural
modification are known in the art and are discussed in U.S. Pat.
Nos. 5,244,650 and 5,208,006.
Members of the ETS-10 molecular sieve zeolite type, have a
crystalline structure and an X-ray powder diffraction pattern with
significant lines at or near those disclosed in Table 1 of U.S.
Pat. No. 5,011,591 the entirety of which is incorporated herein by
reference. By "near" it is meant that the major lines can shift, on
modification of Na-ETS-10, by as much as 1 unit or more, but will
maintain essentially the same pattern in an X-ray powder
diffraction pattern. Hence, modified ETS-10 zeolites will have the
substantially the same pattern of major lines in an X-ray powder
diffraction pattern as unmodified Na-ETS-10.
As prepared ETS-10 zeolites can be prepared by mixing a source of
silica (e.g. silica; silica hydrosol; silica gel; silicic acid;
alkoxides of silicon; alkali metal silicates such as but not
limited to sodium and potassium silicate; mixtures thereof and the
like); a source of trivalent titanium (e.g. TiCl.sub.3 etc.); a
base such as but not limited to an alkali metal hydroxide (e.g.
NaOH, NaOH(aq), KOH, etc.) for controlling the pH of the reaction
mixture at from 9.9 to 10.3.+-.0.1; water; and optionally an alkali
metal halide (NaCl, NaF, KF etc.) in specific ratios. In an aspect
of the invention, Na-ETS-10 is prepared from a reaction mixture
having a composition in terms of mole ratios of:
SiO.sub.2/Ti=from about 2 to about 20
H.sub.2O/SiO.sub.2=from about 2 to about 100
M.sub.n/SiO.sub.2=from about 0.1 to about 10
For further suitable, but non-limiting ratios of these source
components see Table 2 of U.S. Pat. No. 5,011,591 that is
incorporated herein by reference. The mixture is typically heated
to a temperature of between 100.degree. C. and 200.degree. C. and
stirred for at least 8 hours. The "as prepared" Na-ETS-10 zeolite
forms as crystals within the reaction mixture. Stirring of the
reaction mixture is beneficial but in some cases is optional,
especially when using silica gel as the source of silica. The
crystals are separated by filtration and optionally washed with
water, followed by drying at temperatures of up to about
250.degree. C. for up to about 72 hours.
In an aspect of the invention, the "as prepared" or unmodified
Na-ETS-10 is a zeolite prepared according to Examples 5, 6, 7 or 9
of U.S. Pat. No. 5,011,591.
In an aspect of the invention, unmodified Na-ETS-10 zeolite is
prepared and isolated prior to modification by cation exchange
reactions or structural substitution reactions.
Both "as prepared" and "cationically modified" ETS-10 zeolites have
a composition that in some aspects of the invention may be
represented by the following formula: x M.sub.2/nO:TiO.sub.2:y
SiO.sub.2:z H.sub.2O, where M is a mono-, di-, or tri-cationic ion,
n is the valence of M, x is from 1 to 10, y is from 2.5 to 25, and
z is from 0 to 150. In "as prepared" or unmodified titanium
silicate, M is sodium and/or potassium. In cationically modified
ETS-10, sodium and/or potassium ions are ion exchanged for at least
one cation not originally present in the "as prepared" or
unmodified titanium silicate. Alternatively, in cationically
modified ETS-10 zeolites, the Na.sup.+ ions can be replaced with
K.sup.+ ions.
The cation exchange capacity (CEC) is a measure of the exchangeable
cations present in an ETS-10 zeolite. It can be measured in SI
units as the positive charge (in coulombs) absorbed by the zeolite
per unit of mass of the zeolite. It is also conveniently measured
in milliequivalents per gram of zeolite (meq/g) or per 100 gram of
zeolite (meq/100 g). The cation exchange capacity of the unmodified
zeolites is not specifically defined, but in one aspect of the
invention the CEC can be at least 50 millequivalents per 100 g. In
another aspect of the invention, the unmodified zeolite can have a
CEC of from about 1.0 to about 10 meq/g.
The percentage of ions exchanged during the formation of the
cationically modified ETS-10 zeolite is not specifically defined,
provided that the adsorbent remains selective for the adsorption of
olefins over paraffins. By way of a non-limiting example, from
about 5% to 100% of the exchangeable Na.sup.+ and/or K.sup.+ ions
originally present in the "as prepared" ETS-10 may be exchanged by
cation exchange.
In an aspect of the invention, the amount of cation added to the
unmodified ETS-10 can be from about 1% to about 1000% of the
cationic exchange capacity of the zeolite, preferably from about
25% to about 250%. One or more than one type of cationic modifier
can be added to Na-ETS-10. For example, a first cationic modifier
may be added by treating an as prepared Na-ETS-10 zeolite with a
cation in concentrations (meq/g) below the CEC of the zeolite,
followed by the addition of a second, third, or fourth etc.
cationic modifier to replace some or all of the remaining
exchangeable Na.sup.+ and K.sup.+ sites. Cationic exchange can
involve sequential or simultaneous addition of one or more of the
same or different cationic modifiers to an unmodified ETS-10
zeolite.
In the current invention, modification can include partial or full
replacement of exchangeable Na.sup.+ and/or K.sup.+ ions for one or
more than one mono, di- or tri-valent cation or mixture thereof.
Modification can also include partial or complete replacement of
exchangeable Na.sup.+ ions for K.sup.+ ions.
In an aspect of the invention, the modified ETS-10 zeolite is an
"as prepared" ETS-10 zeolite that has been cation exchanged with a
mono, di- or tri-valent cation or mixture thereof. Either or both
of Na.sup.+ or K.sup.+ may be ion exchanged for a mono-, di- or
tri-valent cation.
In an aspect of the invention, the mono-, di- and tri-valent
cations are selected from the group 2-4 metals, a proton, ammonium
compounds and mixtures thereof. Some specific non-limiting examples
of mono-, di, or tri-valent cations that can be used in the current
invention include, Li.sup.+, K.sup.+, Cs.sup.+, Mg.sup.2+,
Ca.sup.2+, Sr.sup.2+, Ba.sup.2+, Sc.sup.3+, Y.sup.3+, La.sup.3+,
Cu.sup.+, Cu.sup.2+Zn.sup.2+, Cd.sup.2+, Ag.sup.+, Au.sup.+,
H.sup.+, NH.sub.4.sup.+, and NR.sub.4.sup.+where R is an alkyl,
aryl, alkylaryl, or arylalkyl group.
The cationic modifiers are generally added to unmodified Na-ETS-10
in the form of a salt or an acid. The anionic counterion associated
with the cationic modifier is not specifically defined, provided
that is does not adversely affect the modification (i.e. cation
exchange) reactions. Suitable anions include but are not limited to
acetate, carboxylate, benzoate, bromate, chlorate, perchlorate,
chorite, citrate, nitrate, nitrite, sulfates, and halide (F, CI,
Br, I) and mixtures thereof. Suitable acids include inorganic and
organic acids, with inorganic acids being preferred.
The Na-ETS-10 zeolite may be cation exchanged by any of the known
conventional techniques. For example, a Na-ETS-10 zeolite may be
cation exchanged by treatment with a cationic modifier in a stirred
aqueous solution. After the cation exchange reactions are carried
out, the resulting modified ETS-10 zeolites can be treated in any
conventional manner, including but not limiting to washing and
drying steps as well as calcinations and granulation steps.
In an aspect of the invention, the modified ETS-10 zeolite is an
"as prepared" ETS-10 zeolite that has been structurally modified.
Either or both of Ti and Si may be substituted by an octahedral
metal and tetrahedral metal respectively.
In an aspect of the invention, titanium is partially substituted by
an octahedral metal selected from the group consisting of but not
limited to arsenic, cobalt, chromium, copper, iron, germanium,
hafnium, magnesium, manganese, molybdenum, niobium, nickel,
antimony, tin, uranium, vanadium, yttrium, zinc, zirconium,
lanthanum, an actinide a lanthanide and mixtures thereof.
In an aspect of the invention, silicon is partially substituted by
a tetrahedral metal selected from the group consisting of but not
limited to aluminum, arsenic, bismuth, boron, beryllium, cobalt,
chromium, copper, iron, gallium, germanium, indium, lead,
magnesium, manganese, molybdenum, niobium, nickel, antimony, tin,
titanium, vanadium, tungsten, zinc and mixtures thereof.
Structurally modified ETS-10 zeolites have a composition that in
some aspects of the invention may be represented by the following
formula: a(1.0.+-.0.25)M.sub.2/nO:AO.sub..alpha.:d BO
.sub..beta.:0-100 H.sub.2O, where M is at least one cation of
valence n; .alpha. is 1/2 the valence of A; .beta. is 1/2 the
valence of B; d is 2-100; a is equal to 1/2 the charge provided by
the total of A and B; A is octahedrally coordinated titanium alone
or a mixture of octahedrally coordinated titanium and another
octahedrally coordinated metal; B is silicon alone or a mixture of
silicon and another tetrahedrally coordinated metal; provided that
when A is only titanium, B cannot be only silicon and that when B
is only Si, A cannot be only Ti.
In an aspect of the invention, structurally modified ETS-10
zeolites are crystalline molecular sieves having a pore size of at
least 8 Angstrom units.
In an aspect of the invention, A is titanium alone or a mixture of
titanium and another metal selected from the group consisting of
but not limited to arsenic, cobalt, chromium, copper, iron,
germanium, hafnium, magnesium, manganese, molybdenum, niobium,
nickel, antimony, tin, uranium, vanadium, yttrium, zinc, zirconium,
lanthanum, an actinide a lanthanide and mixtures thereof.
In an aspect of the invention, B is silicon alone or a mixture of
silicon and another metal selected from the group consisting of but
not limited to aluminum, arsenic, bismuth, boron, beryllium,
cobalt, chromium, copper, iron, gallium, germanium, indium, lead,
magnesium, manganese, molybdenum, niobium, nickel, antimony, tin,
titanium, vanadium, tungsten, zinc, and mixtures thereof.
The Na-ETS-10 zeolite may be structurally modified by any of the
known techniques that are described in for example, U.S. Pat. Nos.
5,208,006 and 5,244,650, which are incorporated herein by
reference. For example, the structurally modified molecular sieves
may be prepared from a reaction mixture containing a source of
titanium or titanium and at least one other metal capable of being
octahedrally coordinated and also containing a source of silicon or
silicon and at least one other metal capable of being tetrahedrally
coordinated in the framework structure, a source of alkalinity such
as an alkali or alkaline earth metal hydroxide, water and,
optionally, an alkali or alkaline earth metal salt.
In an aspect of the invention, a structurally modified ETS-10
zeolite is prepared from a reaction mixture having a composition in
terms of mole ratios of:
B/A=from about 1 to about 200
H.sub.2O/B=from about 1 to about 100
M.sub.n/A=from about 1 to about 100
wherein M indicates the cations of valence n derived from the
alkali or earth metal and metal salts, and A and B are defined as
above.
In an aspect of the invention, a structurally modified ETS-10
zeolite is prepared from a reaction mixture having a composition in
terms of mole ratios of:
SiO.sub.2/Al=from about 1 to about 200
SiO.sub.2/Ti=from about 2 to about 20
H.sub.2O/SiO.sub.2=from about 2 to about 100
M.sub.n/SiO.sub.2=from about 0.1 to about 20
wherein M indicates the cations of valence n derived from the
alkali or earth metal and metal salts. Such, aluminum modified
ETS-10 zeolites have been dubbed, ETAS-10 zeolites (see U.S. Pat.
No. 5,244,650).
The Na-ETS-10 may also be modified by adding a source of metal
which is capable of being octahedrally or tetrahedrally coordinated
within the titanosilicate framework structure, to a previously
prepared Na-ETS-10. By way of non-limiting example, a source of
aluminum (e.g. AlCI.sub.3.6H.sub.2O) may be added to previously
prepared Na-ETS-10 to exchange silicon for aluminum, which is
described in U.S. Pat. No. 5,244,650 (see especially Examples 1-7)
that is incorporated herein by reference. The source of metal which
is capable of being octahedrally or tetrahedrally coordinated in
the framework structure may be stirred and heated with the as
prepared Na-ETS-10 in the presence or absence of solvent or water
to effect Ti and/or Si substitution. Other well known methods for
mixing zeolites with additive compounds may also be used.
The current invention also contemplates modifications that remove
(i.e. extract) a portion of the titanium from an "as prepared"
Na-ETS-10, provided that the framework structure of the Na-ETS-10
remains intact and that the zeolite remains selective for olefin
adsorption. Methods to remove titanium from an ETS-10 type zeolite
are described in U.S. Pat. No. 5,906,954 and include treating the
zeolite with complexing agents such as but not limited to ethylene
diamine tetraacetic acid, oxalic acid and citric acid, amines,
hydroxyl carboxylates and beta diketones.
In an aspect of the invention, the modified ETS-10 zeolite is an
"as prepared" ETS-10 zeolite that has been both cationically
modified and structurally modified.
The modified ETS-10 zeolites used in the current invention can be
used in a wide variety of forms. For example, the modified ETS-10
zeolites may be in the form of a powder, a granule, an extrudate or
other particulate form suitable for use in an adsorbent bed. The
modified zeolites can be mixed with other components prior to use
as an adsorbent most typically in an adsorbent bed. For example,
natural or synthetic clays, aluminophosphates, agglomerates of clay
and silica, silica or other metal oxides, and mixtures thereof may
be added to the modified ETS-10 zeolties.
The modified ETS-10 zeolites can be used with any cycle swing
adsorption process for the separation/enrichment of binary or
multi-component mixtures of olefins and paraffins. For example,
pressure swing adsorption (PSA) processes including vacuum swing
adsorption (VSA), thermal swing adsorption (TSA) processes and
combinations thereof can be used. The cycle swing adsorption
process can comprise multiple adsorption and regeneration steps as
well as purging and depressurization steps. Pressure swing and
temperature swing processes are well known in the art.
Pressure swing adsorption can include, in addition to adsorption
and regeneration steps: purge steps, venting steps, pressure
equalization steps, evacuation steps, blowdown steps. Steps can be
carried out in concurrent, alternating or sequential fashion and
gas flows can be continuous, discontinuous, co-current and
counter-current, all of which are well known in the art. In a PSA
process one or more adsorbent beds can be arranged in series or in
parallel. Some non-limiting examples of PSA processes are described
in Adsorption, Gas Separation in the Kirk-Othmer Encyclopedia of
Chemical Technology, Copyright John Wiley & Sons, Inc. vol 1,
pgs 642-647 and references cited therein as well as in U.S. Pat.
Nos. 3,430,418; 4,589,888; 6,293,999; 6,197,092 and 6,497,750 all
of which are incorporated herein by reference.
Temperature swing adsorption (TSA) is described in Adsorption, Gas
Separation in the Kirk-Othmer Encyclopedia of Chemical Technology,
Copyright John Wiley & Sons, Inc. vol 1, pgs 636-642 and
references cited therein all of which are incorporated herein by
reference.
In an aspect of the present invention, at least one modified ETS-10
adsorbent bed is used in a pressure swing adsorption process to
separate/enrich gaseous mixtures of olefins and paraffins having
the same number of carbon atoms, preferably for the separation of
ethylene from, or the enrichment of ethylene within, a gaseous
mixture containing ethylene and ethane.
In an aspect of the present invention, at least one modified ETS-10
adsorbent bed is used in a pressure swing adsorption process
carried out at ambient temperatures, to separate/enrich gaseous
mixtures of olefins and paraffins having the same number of carbon
atoms, preferably the separation of ethylene from, or the
enrichment of ethylene within, a gaseous mixture containing
ethylene and ethane.
In another aspect of the invention, at least one modified ETS-10
adsorbent bed is used in a combined pressure swing/temperature
swing adsorption process to separate/enrich gaseous mixtures of
olefins and paraffins having the same number of carbon atoms,
preferably the separation of ethylene from, or the enrichment of
ethylene within, a gaseous mixture containing ethylene and
ethane.
The pressures at which adsorption and regeneration steps are
carried out are not specifically defined, and depend on a number of
factors such as but not limited to the temperature used, the type
of cation used to modify the Na-ETS-10 zeolite, the type of
structural modification of the Na-ETS-10 zeolite, and the nature of
the olefin and paraffin to be separated/enriched. Typically, the
range of absolute pressures used during the adsorption step can be
from about 10 kPa to about 2,000 kPa, preferably from about 50 kPa
to about 1000 kPa. The range of pressures used during the release
of adsorbate (i.e. during the regeneration step) can be from about
0.01 kPa to about 150 kPa, preferably from about 0.1 kPa to about
50 kPa.
The temperatures at which the adsorption over the modified ETS-10
zeolite takes place will depend on a number of factors, such as but
not limited to the particular olefin and paraffin to be
separated/enriched, the type of cation used to modify the Na-ETS-10
zeolite, the type of structural modification of the Na-ETS-10
zeolite, and the pressure at which adsorption is to be carried out.
In general, the adsorption step can be carried out at from ambient
temperatures to above about 100.degree. C., provided that the
temperatures do not exceed temperatures at which chemical reaction
of the olefin, such as a polymerization reaction, takes place.
Temperatures that favor adsorption and desorption over the pressure
range of about 0.1 kPa to about 1000 kPa are generally preferred.
For reasons of economics, in one aspect of the current invention,
it is desirable to use ambient temperatures during both the
adsorption and desorption steps.
In an aspect of the current invention, a cationically modified
ETS-10 zeolite has a superior pressure swing capacity for ethylene
and ethane adsorption/desorption, in the pressure range of about 1
kPa to about 200 kPa, than an unmodified Na-ETS-10 zeolite.
In an aspect of the current invention, a cationically modified
ETS-10 zeolite has a pressure swing capacity of at least about 0.5
mmol/g, preferably at least about 1.0 mmol/g for ethylene, in the
pressure range of about 1 kPa to about 200 kPa, at a temperature of
about 25.degree. C.
In an aspect of the current invention, a modified ETS-10 zeolite is
used to selectively adsorb ethylene from a gaseous feedstream
containing ethylene and ethane, to produce an adsorbed phase
enriched in ethylene and a non-adsorbed phase enriched in ethane.
Desorption from the modified ETS-10 zeolite occurs at a pressure
which is lower than the adsorption pressure, and a gaseous mixture
rich in ethylene is recovered as product or may be further enriched
by further treatment with modified ETS-10 zeolite. The feedstream
may optionally contain gases such as carbon monoxide, carbon
dioxide and hydrogen. However, it is preferable to treat the
feedstream to remove carbon monoxide, carbon dioxide and hydrogen,
prior to contact with the modified ETS-10 adsorbent. Components
such as hydrogen sulfide may also be present in the feedstream and
are preferably removed prior to contact with the adsorbent. Methods
to remove hydrogen, hydrogen sulfide, carbon monoxide etc. are well
known in the art.
In the current invention, the modified ETS-10 zeolite can be used
in a pressure swing adsorption (PSA) process that receives product
feedstreams from a hydrocarbons cracking unit or plant. A
hydrocarbon cracking unit typically employs hydrothermal pyrolysis
or high temperature catalytic processes to crack feedstocks such as
but not limited to natural gas, naphtha and gas oil, for the
production of light olefins such as ethylene and propylene.
Preferred cracking processes include stream cracking of ethane to
form ethylene, as used in a conventional ethane hydrocracking
plant.
The methods and processes of the current invention can be used in a
variety of petroleum refining and petrochemical operations where
the separation of ethylene/ethane product streams is desired. For
example, the current process can be used to perform a rough
separation of ethylene and ethane prior to cryogenic fractionation
of ethylene and ethane or alternatively to perform a final
purification or finishing step after a rough cut distillative
separation of ethylene and ethane. Cryogenic fractionation of
ethylene from ethane is well known in the art. The generation of a
C2 feedstream from the products of hydrocracking is also well known
in the art and principally involves compression, acetylene
hydrogenation, de-methanization, and various fractionation steps to
remove higher olefins and higher paraffins.
In the present invention, modified ETS-10 zeolites can be used in
one or more PSA beds, upstream of a ethylene/ethane distillation
unit (i.e. a C2 splitter column). Alternatively, one or more PSA
beds containing modified ETS-10 can be downstream of an
ethylene/ethane distillation unit.
Without wishing to be bound by any single theory, use of PSA units
containing modified ETS-10 zeolites can augment the separation
performance of a C2 splitter column by increasing the proportion of
olefins in a mixture of olefins and paraffin having the same number
of carbon atoms. As a result, the investment and energy
requirements for ethylene/ethane cryogenic separation, such as for
example, the size of the C2 splitter column, may correspondingly be
reduced.
EXAMPLES
Unmodified ("As Prepared") Na-ETS-10 Zeolites
Unmodified ETS-10 was synthesized hydrothermally as described in
U.S. Pat. No. 5,011,591. A typical preparation involved thorough
mixing of 50 g of sodium silicate (28.8% SiO.sub.2, 9.14% Na.sub.2O
obtained from Fisher Scientific), 2.3 g of sodium hydroxide
(97.sup.+% NaOH, obtained from Fisher Scientific), 3.8 g of
anhydrous KF (Fisher Scientific), 4 g of HCl (1M aqueous solution),
and 16.3 g of TiCl.sub.3 solution (30 wt. % Solution in 2N
Hydrochloric Acid, from Fisher Scientific). The mixture was stirred
in a blender for 1 h and then placed in a 125 mL sealed autoclave
(by PARR Instruments) at 488 K for 64 h. This gave a resultant
material that was washed with de-ionized water and dried in an oven
at 373 K. The material could be extruded into a less than 100 mesh
(<150 .mu.m) powder.
Preparation of Modified ETS-10 Zeolites by Cation Exchange
Cation-exchange was carried out by exposing Na-ETS-10 material
prepared as above (a less than 100 mesh powder) to an excess of
aqueous ionic solution at 100.degree. C. with stirring for 24 h.
The aqueous ionic solutions added were alternatively an aqueous
solution of LiCl, KCl, BaCl.sub.2, AgNO.sub.3 and CuSO.sub.4. The
exchanged materials were washed with de-ionized water and dried at
100.degree. C.
Mixed cationic forms of ETS-10 including Ba/H and La/H forms were
prepared by exposing Na-ETS-10 powder to 1 meq/g of an aqueous
solution of BaCl.sub.2 or LaCl.sub.3 at 100.degree. C. with
stirring for 16 h. This provided partially exchanged materials that
were then exposed to an HCl solution maintained at a pH of 2 for 8
h at 20.degree. C. The final products were washed with de-ionized
water and dried at 100.degree. C.
Ethylene and Ethane Adsorption Studies
Ethylene and ethane adsorption isotherms were measured in a
Rubotherm magnetic suspension balance (accuracy.+-..mu.g),
integrated into a GHP high pressure adsorption system constructed
by VTI Corp. of Hialeah, Fla. Test samples were dried at
200.degree. C. for 6 h under a vacuum of more than 10.sup.-4 Torr.
Buoyancy effects were corrected with a helium displacement isotherm
taken at the same temperature as the respective ethylene and ethane
isotherms.
Nitrogen adsorption isotherms at -196.degree. C. were measured in
an AUTOSORB-1 volumetric system from Quantachrome Instruments,
Boynton Beach Fla. Nitrogen isotherms for all the modified ETS-10
samples are type I according to the IUPAC classification (see
Rouquerol et. al. in Adsorption by Powders and Porous Solids:
Principles, Methodology and Applications. Academic Press, San
Diego, Calif.). Equivalent specific surface (S.sub.total) was
calculated by applying the BET equation, and external surface
(S.sub.ext), internal surface (S.sub.int) and micropore volume
(V.sub.mic) were calculated by the V-t method (see Greg, S. J;
Sing, K. S. W. in Adsorption, Surface Area and Porosity. 1982
Academic Press, London-New York and Rouquerol, F. Rouquerol, J.;
Sing, K. S. W. in Adsorption by Powders and Porous Solids:
Principles, Methodology and Applications. Academic Press, San Diego
Calif.). Surface analysis results are given in Table 1.
TABLE-US-00001 TABLE 1 Surface Analysis for Unmodified Na-ETS-10
and Modified ETS-10 Adsorbent S.sub.total (m.sup.2/g) S.sub.ext
(m.sup.2/g) S.sub.int (m.sup.2/g) V.sub.mic (cc/g) Na-ETS-10 289 28
261 0.099 K-ETS-10 178 28 150 0.067 Li-ETS-10 321 22 299 0.123
Ba-ETS-10 350 35 315 0.119 Ba/H-ETS-10 417 30 387 0.146 La/H-ETS-10
420 26 394 0.151 Ag-ETS-10 209 19 190 0.071 Cu-ETS-10 189 45 144
0.056
FIGS. 1a-1h shows the ethylene and ethane adsorption isotherms for
unmodified Na-ETS-10 (FIG. 1a) and various cationically modified
ETS-10 zeolites (FIGS. 1b-1h) at 25.degree. C. A person skilled in
the art will recognize that the isotherms for Na-ETS-10 and
K-ETS-10 are rectangular in shape, consistent with a low pressure
swing capacity. The isotherms for ETS-10 modified with Ag is very
rectangular, indicative of irreversible adsorption and poor
pressure swing capacity. In contrast, isotherms for ETS-10 modified
with Li, Ba, Ba/H, La/H and Cu have greater curvature, consistent
with an improved pressure swing capacity.
Modeling (Pure Component Isotherms and Binary Ethylene/Ethane
Mixture)
Model analysis of ethylene and ethane adsorption isotherms for the
modified and unmodified ETS-10 zeolites were carried out by
following the procedure described by Al-Baghli and Loughlin in J.
Chem. Eng. Data 2005, v 50, p 843 and J. Chem. Eng. Data 2006, v
51, p 248, which are incorporated herein by reference. Experimental
isotherms for ethylene and ethane adsorption were fitted using the
Toth equation: n=n.sub.mp/(b+p).sup.1/t, where n is the amount of
ethylene or ethane adsorbed (in mmol) at a pressure, p (in
kilopascals, kPa), n.sub.m is the monolayer adsorption capacity of
the adsorbent (in mmol/g), t can vary from 0 to 1, and b is related
to the Henry's law constant, K by the expression:
K=n.sub.mb.sup.-1/t. The selectivity, .alpha. of the adsorbent, in
the Henry's law region is defined as the ratio of the Henry's law
constants of the pure gas components:
.alpha.=K.sub.ethylene/K.sub.ethane
In order to determine physically meaningful fitting values for
monolayer adsorption capacity, n.sub.m and the selectivity, .alpha.
the magnitude of n.sub.m, was theoretically calculated, while the
parameters b and t could vary during the fitting to the
experimental data. To fit the equation, t and b were allowed to
vary (with t varying from 0 to 1) until optimum parameters were
obtained, using any well known fitting or regression technique,
such as but not limited to the least squares technique, using the
n.sub.m value determined as below. This "constrained regression"
method for the Toth equation fitting is described by Al-Baghli and
Loughlin in J. Chem. Eng. Data, 2005, v 50, p 843 which is
incorporated herein by reference.
The monolayer adsorption capacity is determined using the equation:
n.sub.m=0.95(.epsilon./V*), where .epsilon. is the monolayer volume
of the adsorbent (in cc/g), V* is the molar volume (in cc/mol) of
the adsorbate at the temperature of the isotherm measurement, and
the factor 0.95 accounts for steric effects (see Al-Baghli and
Loughlin in J. Chem. Eng. Data, 2005, v 50, p 843). The value for
.epsilon. can be calculated directly from the nitrogen isotherm at
-196.degree. C. by applying the BET equation according to known
methods. The value for .epsilon. can also be obtained form the
surface parameters given in Table 1 using the equation:
.epsilon.=(S.sub.total/S.sub.int)V.sub.mic, where S.sub.total,
S.sub.int and V.sub.mic are defined as above. The molar volume, V*
can be approximated by the Van der Waals volume, which is 0.055
cc/mol for ethylene and 0.063 cc/mol for ethane under supercritical
conditions for the adsorbable gases.
The "constrained" Toth equation fits well with most of the
experimentally determined ethylene and ethane adsorption isotherms
in the pressure range of from 1 to 200 kPa for the modified ETS-10
zeolites (see FIGS. 1a-1h). Some deviation between the model and
the experimental data is observed in the case of Li-ETS-10.
Model predictions for binary ethylene/ethane adsorption isotherms
were carried out by applying the ideal adsorption solution theory
(IAST) developed by Myers and Prausnitz in Thermodynamics of Mixed
Gas Adsorption, A.I.Ch.E. Journal, vol 11, No 1, pg 121 and as used
by Al-Baghli and Loughlin in J. Chem. Eng. Data 2006, v 51, p 248,
which are incorporated herein by reference. The algorithm proposed
by Valenzuela and Myers in the Adsorption Equilibrium Data
Handbook, 1989, Prentice Hall, Englewood Cliffs, N.J., which is
incorporated herein by reference, was used to for the IAST
mathematical analysis.
The "constrained" Toth equation was used as a model to generate
pure compound isotherms for the IAST calculations. The constrained
Toth parameters used for the IAST calculations are given in Tables
B and C.
TABLE-US-00002 TABLE 2 Constrained Toth Parameters for Adsorption
of Ethylene at 25.5.degree. C. Adsorbent n.sub.m (mmol/g) b
(kPa.sup.t) t K (mmol/g kPa) Na-ETS-10 1.89 0.24 0.37 89.45
K-ETS-10 1.37 0.42 0.59 5.96 Li-ETS-10 2.28 0.51 0.50 8.77
Ba-ETS-10 2.28 0.86 0.48 3.12 Ba/H-ETS-10 2.72 2.12 0.53 0.66
La/H-ETS-10 2.78 3.36 0.56 0.32 Ag-ETS-10 1.35 0.29 0.33 57.47
Cu-ETS-10 1.27 1.89 0.53 0.38
TABLE-US-00003 TABLE 3 Constrained Toth Parameters for Adsorption
of Ethane at 25.5.degree. C. Adsorbent n.sub.m (mmol/g) b
(kPa.sup.t) t K (mmol/g kPa) Na-ETS-10 1.65 0.54 0.40 7.70 K-ETS-10
1.20 1.04 0.62 1.13 Li-ETS-10 1.99 1.37 0.63 1.21 Ba-ETS-10 1.99
1.71 0.58 0.79 Ba/H-ETS-10 2.37 3.25 0.57 0.30 La/H-ETS-10 2.43
3.36 0.55 0.27 Ag-ETS-10 1.18 0.58 0.41 4.46 Cu-ETS-10 1.11 2.01
0.47 0.25
The IAST selectivity, .alpha..sub.ij of the adsorbent, for the
adsorption of ethylene from a mixture of ethylene and ethane was
calculated at a given total pressure using the following equation:
.alpha..sub.ij=y.sub.ix.sub.j/y.sub.jx.sub.i, where, x.sub.i,
y.sub.i are the molar fractions of ethane in the adsorbed phase and
the gas phase respectively, and x.sub.j and y.sub.j are the molar
fractions of ethylene in the adsorbed phase and gas phase
respectively.
FIGS. 2a and 2b show the Ideal Adsorption Solution Theory (IAST)
ethylene/ethane selectivity at 25.degree. C., as a function of the
total pressure, for unmodified and cationically modified ETS-10
zeolites (y is equal to 0.5). The IAST model shows that the
ethylene/ethane selectivity generally increases with pressure. A
person skilled in the art will recognize that modification with Li,
Cu, Ba, BaH, and La/H leads to lower IAST selectivity than for
unmodified Na-ETS-10 zeolites. However, comparison of FIGS. 1 and 2
shows that a good balance of IAST selectivity and pressure swing
capacity can be obtained by suitable modification.
FIG. 3 shows the IAST plot of the molar fraction of ethylene in the
gas phase (Y.sub.ethylene) vs. the molar fraction of ethylene
adsorbed (X.sub.ethylene) for various modified ETS-10 zeolites, at
25.degree. C. and a total pressure of 150 kPa; (a) as prepared
Na-ETS-10; (b) K, (c) Li, (d) Cu and Ba (e) Ba/H and (f) La/H
cation modified ETS-10 zeolites.
* * * * *